The present invention relates to an active matrix substrate that can be used for flat panel type display devices such as a liquid crystal display device (LCD), an electrochromic display device (ECD) and an electroluminescent display device (ELD) and flat panel type image sensors such as an X-ray image sensor and a contact type image sensor, a method for fabricating the active matrix substrate and a liquid crystal display device that employs the active matrix substrate.
In an active matrix substrate to be used for flat panel type display devices represented by the LCD, electrode wiring lines (scanning electrodes and signal electrodes) are provided in a matrix form on an insulating substrate, and an active element (thin film transistor (TFT), for example) and a pixel electrode are provided at each intersection of the lines.
FIG. 4 is a longitudinal sectional view showing a representative structure of an active matrix substrate to be used for a transmission type LCD. With regard to this active matrix substrate, a pixel electrode 9 is normally constructed of a transparent conductive film made of ITO (Indium Tin Oxide). In the above case, ITO is formed by a vacuum film forming technique of sputtering or the like.
In FIG. 4 are shown a glass substrate 1, a scanning electrode 2, a gate electrode 3 connected to the scanning electrode 2, a gate insulating film 4, a channel layer 5, a contact layer 6, a source electrode (signal electrode) 7, a drain electrode 8 and an insulative protecting film 10. Then, the gate electrode 3, gate insulating film 4, channel layer 5, contact layer 6, source electrode 7 and drain electrode 8 constitute a TFT 11.
However, the aforementioned conventional active matrix substrate has the problems as follows. That is, in order to pattern the transparent conductive film formed as described above by the vacuum film forming technique into the shape of the pixel electrode 9, it is required to carry out an ITO film forming process comprised of (1) film formation by sputtering, a photoresist patterning process comprised of (2) resist coating, (3) pre-bake, (4) exposure, (5) development and (6) post-bake and an etching and resist stripping process comprised of (7) etching, (8) resist stripping and (9) cleaning, as shown in FIG. 5, and this means that many processes from the (1) film formation by sputtering through to the (9) cleaning are required.
Lately, an increasing number of LCD""s are used for a number of devices such as computers, portable terminal devices, television sets and so on, and it is expected that the flat panel type display devices will have an increasing demand in future. As concerns the increasing demand, it is required to reduce the cost of the active matrix substrates to be used for the flat panel type display devices.
However, the conventional active matrix substrate needs many processes for the reason that the transparent conductive film formed by the vacuum film forming technique is subjected to patterning when forming the pixel electrodes 9 as described hereinabove, and this impedes the achievement of the reduction in cost of the active matrix substrates.
Accordingly, the object of the present invention is to provide an active matrix substrate capable of remarkably reducing the fabricating processes of the pixel electrodes, a method for fabricating the active matrix substrate and a liquid crystal display device.
In order to achieve the above object, according to the first inventive aspect, there is provided an active matrix substrate having electrode wiring lines arranged in a matrix form, a plurality of active elements provided at intersections of the electrode wiring lines and a plurality of pixel electrodes connected to the electrode wiring lines via the active elements on an insulating substrate, wherein
the pixel electrodes are formed of a transparent conductive oxide film made of a sol-gel material.
According to the above construction, a sol-gel material capable of being coated on the substrate by the spin-coating method or the dipping method is employed as a pixel electrode material. Therefore, transparent pixel electrodes can be formed without using any vacuum film forming apparatus, and the fabricating processes of the pixel electrodes are reduced.
In one embodiment, no constituent member of the electrode wiring lines and the active elements exists between the pixel electrodes and the active matrix substrate.
In one embodiment, the pixel electrodes are formed in a process preceding processes of forming the electrode wiring lines and the active elements.
According to the above construction, no constituent member of the electrode wiring lines and the active matrix elements exists on the substrate in the pixel electrode forming stage. Therefore, the pixel electrode can be formed without regard to the heat resistance temperatures of these components. Accordingly, the transparent electrode oxide film formed of the sol-gel material that requires high-temperature baking at a temperature of about 500xc2x0 C. can be employed as a material for the pixel electrodes.
In general, the sol-gel material requires high-temperature baking at a temperature of about 500xc2x0 C. in the film forming stage for the removal of the residual organic matter. However, according to the above construction, the pixel electrodes are firstly formed, and the electrode wiring lines and the active elements are subsequently formed. Therefore, even if the heat resistance temperature of the electrode wiring lines and the active elements is about 300xc2x0 C., an active matrix substrate is formed without inflicting thermal damage to the components. In this case, if the electrode wiring lines and the active elements are firstly formed, then there occurs the problem that the metal constituting the electrode wiring lines and the semiconductor of a-Si (amorphous silicon) or the like constituting the active elements are disadvantageously deteriorated in the high-temperature baking stage of the sol-gel material.
If the high-temperature baking is performed at a temperature of about 500xc2x0 C. during the active matrix substrate fabricating processes, then the substrate is sometimes slightly deformed in size in accordance with the expansion and contraction reactions of the foundation glass substrate. If such a high-temperature process exists in the active matrix substrate fabricating processes, then there occurs the problem that the pattern superposition accuracy is degraded before or after the process. However, in the case of the aforementioned construction, the electrode wiring lines and the active elements are formed after the formation of the pixel electrodes. Therefore, even if the substrate is deformed in size in the pixel electrode forming stage, the pattern superposition accuracy of the pixel electrodes, the electrode wiring lines and the active elements is not degraded by superposing the patterns of the electrode wiring lines and the active elements on the basis of the pixel electrode pattern.
In one embodiment, the pixel electrodes are treated with heat at a temperature higher than those of the electrode wiring lines and the active elements.
According to the above construction, the transparent conductive oxide film formed of the sol-gel material is treated with heat at a temperature (about 500xc2x0 C.) higher than the temperature (the maximum temperature is normally about 350xc2x0 C.) at which the electrode wiring lines and the active elements are formed. Therefore, the residual organic matter in the sol-gel material can be sufficiently decomposed and removed, and this allows a high-quality transparent conductive oxide film to be obtained.
In one embodiment, the pixel electrodes are principally made of any one of indium tin oxide, tin oxide, indium oxide, zinc oxide, germanium oxide and titanium oxide or a mixture of these substances.
The metal oxides of ITO, SnO2, indium oxide, zinc oxide, germanium oxide and titanium oxide, which can be easily formed by the sol-gel method and exhibit transparency and conductivity, are appropriate as a material for the pixel electrode.
In particular, the substances of ITO and SnO2 can be easily provided with a corrosion resistance to chemical liquids and gases necessary for the processes of processing the other members constituting the active matrix substrate. For example, the substances of ITO and SnO2 have a resistance to chemical liquids and gases (etching liquid and etching gas of metal and semiconductor materials, photoresist, developing liquid, resist stripper, substrate cleaning liquid and so on) other than some acids such as HCl and HBr. According to the aforementioned construction, the substances of ITO and SnO2 or these substances doped with Sb are used as principal materials. This accordingly makes it easy to firstly form the pixel electrodes and subsequently form the electrode wiring lines and the active elements. Furthermore, doping the substances of ITO and SnO2 with Sb also enables the reduction in resistance of the transparent conductive oxide film.
The substance of SnO2, which has a strong corrosion resistance and is hard to be patterned, has not conventionally been employed for the pixel electrodes of the active matrix substrate. However, employing the sol-gel material that has photosensitivity as described above facilitates patterning and enables the application of the material to the pixel electrodes. The substance of SnO2 has a transmittance to visible light superior to that of the aforementioned ITO. Therefore, by employing the substance of SnO2 for the pixel electrodes, an active matrix substrate having an excellent transmittance is formed.
According to the second inventive aspect, there is provided an active matrix substrate fabricating method for fabricating an active matrix substrate having electrode wiring lines arranged in a matrix form, a plurality of active elements provided at intersections of the electrode wiring lines and a plurality of pixel electrodes connected to the electrode wiring lines via the active elements on an insulating substrate, comprising the step of:
forming the pixel electrodes of a sol-gel material in a process preceding processes of forming the electrode wiring lines and the active elements.
According to the above construction, by adopting the sol-gel material capable of being coated as a pixel electrode material by the spin-coating method or the dipping method, the vacuum film forming apparatus is not required to be used, and the pixel electrode fabricating processes are reduced, similarly to the aforementioned first inventive aspect. Furthermore, the electrode wiring lines and the active elements are formed after the formation of the pixel electrodes. Therefore, even if the heat resistance temperature of the electrode wiring lines and the active elements is about 300xc2x0 C., an active matrix substrate is formed without inflicting thermal damage to the components.
According to the third inventive aspect, there is provided an active matrix substrate fabricating method for fabricating an active matrix substrate having electrode wiring lines arranged in a matrix form, a plurality of active elements provided at intersections of the electrode wiring lines and a plurality of pixel electrodes connected to the electrode wiring lines via the active elements on an insulating substrate, comprising the step of:
forming the pixel electrodes by patterning a sol-gel material having photosensitivity.
According to the above construction, the sol-gel material employed as a material for the pixel electrodes has photosensitivity. Therefore, neither the photoresist patterning process nor the etching process is needed in patterning the transparent conductive oxide film into the pixel electrode shape, and the processes are reduced further than in the case of the conventional patterning with photoresist. Therefore, the pixel electrode fabricating processes are reduced.
In one embodiment, a chelating agent for imparting photosensitivity is added to the sol-gel material.
If the gel film is formed by using the metal alkoxide chemically modified with the chelating agent, the gel film has a resolution significantly changed by ultraviolet irradiation. That is, the gel film to which ultraviolet rays are irradiated has its chelate bond severed and becomes insoluble to alkaline solutions and alcohol. According to the above construction, the chelating agent is added to the sol-gel material employed as the material of the pixel electrodes. Therefore, photosensitivity is easily imparted to the sol-gel material.
In one embodiment, a photosensitive resin for imparting photosensitivity is added to the sol-gel material.
According to the above construction, the sol-gel material employed as the material of the pixel electrodes is blended with the photosensitive resin at an appropriate ratio, by which photosensitivity is easily imparted to the sol-gel material. For example, if ultraviolet rays are irradiated to a precursor film of a material obtained by blending the sol-gel material with a monomer having photopolymerizability and a polymerization initiator, the monomer is polymerized to form a reticulated polymer network, where the sol-gel material exists in the spaces of the polymer network. Therefore, by subsequently performing a development process, only the film of the polymerized portion to which ultraviolet rays have been irradiated remains as a negative pattern, and the sol-gel material in the portion that has not undergone irradiation is dissolved in the developing fluid together with the unpolymerized monomer.
According to the fourth inventive aspect, there is provided a liquid crystal display device including any one of the above active matrix substrates.
According to the above construction, the active matrix substrate of which the pixel electrode fabricating processes are reduced for the achievement of cost reduction is employed to achieve cost reduction of the liquid crystal display device.